1. Field of the Invention
The invention relates to an optical device for scanning an optical record carrier, and to an optical grating for use in such a device. More particularly, the device includes a radiation source for generating a first radiation beam of a first wavelength traveling along a first path and a second radiation beam of a different, second wavelength traveling along a different, second path, a photo-detection system, an optical system for guiding the radiation beams via the optical record carrier to the photo-detection system, and a grating for combining the first and second beams such that they substantially coincide on the photo-detection system. Scanning may refer to reading, writing and erasing information from the record carrier.
2. Description of the Related Art
The increasing demand for storage capacity has led to the development of new optical scanning devices and matching record carriers having an increased storage capacity. As a consequence, old, low capacity record carriers and new, high capacity record carriers are simultaneously available on the market. For compatibility reasons, a scanning device for a new record carrier should be able to scan both old and new record carriers. This requires adaptation of the device to handle the different formats of the record carriers. As an example, a scanning device designed for scanning both the newer record carrier of the DVD type and the older writable record carrier of the CD-R type must form a 650 nm wavelength radiation beam for scanning the DVD and a 780 nm wavelength radiation beam for scanning the CD. The optical system of such a player includes two diode lasers, one for 650 nm radiation and one for 780 nm radiation. To reduce the cost of the optical system, as many of its components as possible should be traversed by both radiation beams.
Such a dual wavelength scanning device is known from U.S. Pat. No. 5,912,868 and is schematically presented in FIG. 1. Paths 1 and 2 of the two radiation beams from lasers 3 and 4 are mutually oriented under 90° and combined in a cube beam splitter 5 before entering an objective system 6 that focuses the beams on a record carrier 7. The choice of which radiation source is operated is determined by the type of record carrier being scanned. Radiation returning from the record carrier is guided to a photo-detection system 8 via a beam splitter 9. The photo-detector transforms the impinging radiation into electrical signals that represent information stored on the record carrier and tracking information indicating the positional accuracy of the focus of the radiation beam on the tracks of the record carrier on which the information is written. The tracking relates to tracking in a direction of the optical axis, i.e., focusing, and the tracking in a direction perpendicular to both the optical axis and the direction of a track being scanned in the record carrier. The latter type of tracking is also referred to as radial tracking where it relates to disc-shaped optical record carriers.
The tolerance in the mutual position of the two lasers is relatively tight in view of the accuracy with which the radiation beams must fall on the detection system. Errors in the position on the detection system may cause errors in the tracking information. FIG. 2 shows a scanning device known from Japanese Patent Application No. JP-A 10326428, in which the two lasers are arranged close together, drawn as a single component 10, making it easier to keep their mutual position within the tolerance. The paths of the two radiation beams are at an acute angle, and the beams are combined by a grating 11. The grating diffracts both incident radiation beams in transmission. The zeroth-order beam of one of the radiation beams passes from the grating to the objective system, whereas the first-order beam of the other radiation beam passes along the same path to the objective system.
Japanese Patent Application No. JP-A 10261241 discloses a grating for combining the radiation beams from the lasers. The grating is optimized for transmission of the radiation of 650 nm wavelength in the zeroth order and radiation of 780 nm in the first order. The ruling of the grating is in the form of a series of adjacent saw-tooth profiles, each saw-tooth being approximated by a stepped profile.